1. Field of the Invention
This invention relates to an overcurrent protection apparatus in a switching power supply, and more particularly to an overcurrent protection apparatus for protecting a power switching element from damage, which element is employed in a switching power supply.
2. Description of the Prior Art
A stabilized power supply of a continuously controlled type ordinarily invites losses in a control circuit and therefore is unsuitable for a stabilized power supply of the type that supplies high power. On the other hand, a switching power supply is not accompanied by large control circuit losses and for this reason is well-suited for application as a stabilized power supply of the type described.
A switching power supply generally includes a rectifier circuit for rectifying AC voltage, a transformer having a primary winding connected to the rectifier circuit and a secondary winding connected to a smoothing circuit, and a power switching element for opening and closing the primary side of the transformer, with DC voltage of a prescribed level being obtained from the output of the smoothing circuit by suitable switching control of the power switching element.
A power switching element in the switching power supplies of the aforesaid type is likely to be damaged if an excessive current flows into it. This damage to the power switching element owing to the inflow of excessive current occurs when the iron core of the transformer saturates, when a short-circuit occurs in the secondary wiring for some unforeseen reason, or when the associated circuitry is overloaded. It is therefore common practice in the conventional switching power supplies to provide the primary circuit with current detection means operable to detect the value of the primary current, in which the arrangement is such that an excessive primary current is not permitted to flow into the power switching element because the element is turned off when the primary current is detected as exceeding a predetermined value. However, an important drawback in this conventional overcurrent protection apparatus is a delay between detection of primary current and the opening of the power switching element. The delay is not a major factor as long as the increase in primary current defines a comparatively gentle curve, in which case the power switching element can be almost fully protected. The problem arises when the primary current increases abruptly due to an accident such as saturation of the transformer core or a short-circuit on the secondary side, in which case the delay mentioned above does not allow the power switching element to be protected until after the excessive current has begun to flow. The power switching element suffers damage as a result. A method of preventing such damage in the conventional overcurrent protection apparatus involves the use of a power switching element of a large current capacity, but the cost of these switching elements increases in proportion to their current capacity, and the amount of heat which is generated does not dissipate easily.
Another problem encountered in the conventional overcurrent protection apparatus involves the supply of the DC voltage. Once an overcurrent has been detected the power switching element is turned off to protect it and then, when the cause of the overcurrent has been eliminated, the switching of the element resumes in normal fashion so that the supply of the DC voltage can continue. However, the supply of the DC voltage also is terminated even if the primary current accidentally becomes excessive only for a brief instant and then returns to a normal value. This unnecessary interruption in DC voltage diminishes operating efficiency since it may deprive the DC-powered equipment such as a numerical control equipment of its driving voltage and hence cause the equipment to cease operating. Moreover, although an excessive inrush current generally flows into the primary circuitry when an AC power source is connected, it is necessary that the conventional overcurrent protection apparatus be designed so that it will not react to this inrush current. Such a requirement complicates the circuitry and raises the cost.
A further difficulty in the prior-art apparatus involves the fact that there are occasions where the switching elements in a switching power supply are turned on for a longer period of time than they are turned off, which means that the energy stored in the inductance of the transformer when the switching elements are on is not completely released. The remaining energy gradually accumulates, causes the transformer to saturate, and leads to an excessive primary current. This in turn invites damage to the elements and an interruption in DC voltage.